1. Field of the Invention
The present invention relates to a method of producing a hot-dip galvannealed steel sheet. More particularly, the present invention is concerned with a method of preventing generation of white stripe defect which is often exhibited in production of hot-dip galvannealed steel sheet by using Ti-containing very-low-carbon cold-rolled steel sheet as the base material. This invention further provides a hot-dip galvannealed steel sheet having excellent corrosion resistance, weldability and press-workability.
2. Description of the Related Art
Hot-dip galvanized steel sheets generally exhibit superior corrosion resistance and, hence, have been used extensively in various fields. In particular, hot-dip galvannealed steel sheet, in which Fe of the base steel sheet has been diffused in the plating layer so as to be alloyed with Zn through a galvannealing subsequent to hot-dip galvanizing, exhibit superior spot-weldability and formability, as well as excellent corrosion resistance after painting, as compared with hot-dip galvanized steel sheet which has not been subjected to galvannealing. Due to such superior properties, hot-dip galvannealed steel sheets are finding spreading use, in particular in the field of inner and outer panels of automobiles.
Hitherto, very-low-carbon cold-rolled steel sheets, which excel in deep-drawability, have been used as the material of members which have to undergo a very severe forming work, e.g., inner and outer panels of automobiles. Some very-low-carbon cold-rolled steel sheets have been known having a basic composition obtained by minimizing the C content which impedes deep drawability and the minimized C content is fixed by addition of a trace amount of Ti with additive elements added for the purpose of improving strength and weldability.
An attempt has been made to produce a surface-treated steel sheet superior both in deep-drawability and corrosion resistance, by using a very-low-carbon cold-rolled steel sheet of the type mentioned above. For instance, Japanese Patent Laid-Open No. 1-184227 discloses a method of producing a hot-dip galvannealed steel sheet having excellent drawability.
Hitherto, a Sendzimir type plating line has been used in the production of hot-dip galvannealed steel. In this line, a steel sheet is quickly heated in a non-oxidizing furnace to burn and remove any grease on the steel sheet and is then subjected to a heating reduction conducted in a reducing atmosphere of an inert gas such as H.sub.2 and N.sub.2. The steel sheet is then introduced into a hot-dip galvanizing bath in which it is hot-dip galvanized. Then, the coating weight is adjusted through a gas-wiping and is heated by, for example, a cup burner so as to be alloyed, whereby a hot-dip galvannealed steel sheet is obtained.
According to this method, the steel sheet is made to pass through a non-oxidizing furnace which serves as a preheating furnace, prior to the heating reduction and annealing. Any grease on the steel sheet is removed by burning as the steel sheet passes through the non-oxidizing furnace, so that both pre-heating and cleaning of the steel sheet are conducted simultaneously to offer a high economy.
It has been found, however, a treatment of the aforementioned Ti containing very-low-carbon cold-rolled steel sheet in the above-mentioned plating line having a non-oxidizing furnace exhibit a surface defect in the form of white stripes which seriously degrade the appearance of the product sheet.